Alkanolamines are widely applied as solvents in the oil and chemical industry, for instance in absorption or extraction processes. Aqueous solutions of these compounds are much used to remove acidic compounds such as hydrogen sulphide or carbon dioxide from gas streams. Usually these removal processes are carried out regeneratively. This means that after the removal of acidic compounds from a gas stream by contacting the gas stream with a solvent in an absorption column at a relatively low temperature, yielding a purified gas and a so-called rich (or fat) solvent, the rich solvent is led to a regeneration column. There the rich solvent is heated and the acidic components are desorbed from the solvent, thus yielding an off-gas consisting of acidic components and a so-called lean solvent which may be recycled to the absorption column.
It is known that such solvents gradually degrade under the influence of heat, air, etc. So the heating applied during the regeneration, i.e. the desorption step, may cause some degradation. Also, certain absorbed acidic compounds may react with alkanolamines to form amine salts which are not desorbed from the alkanolamine by the heat in the regeneration column. In the art, such salts are generally called heat stable salts. Other contaminants often found are amides and oxazolidones. Oxazolidones may be formed by the reaction between an alkanolamine and carbon dioxide. For instance, the reaction of CO.sub.2 with di-isopropanolamine yields 3-(2-hydroxypropyl)-5-methyl-2-oxazolidone (often just referred to as "oxazolidone"). In small concentrations such contaminants and degradation products do not present any operational problems, but in larger concentrations they do.
Firstly they cause a lowering of the molarity of the alkanolamine in the total solvent system and thus reduce the effectiveness of the absorption process. Furthermore it has been found that certain heat stable salts interfere with the normally occurring passivation of the carbon steel which is preferably employed for the absorption and regeneration columns and their internals and interconnections. This in turn causes corrosion and fouling by ferrous sulphide. Replacing the carbon steel by corrosion resistant steel is less attractive, for obvious cost reasons.
Another operational problem caused by heat stable salts and other contaminants in alkanolamine based solvent systems is foaming. The causes hereof are not yet completely understood, though it has been found that foaming occurs hardly or not at all if the solvent is clean. Many operators find it advantageous to monitor the concentration of the contaminants in the alkanolamine-water mixture, especially the heat stable salts, and to keep them at a low level, of e.g. less than 3000 ppm heat stable salts as anions.
In the past a commonly accepted practice was to continuously refresh a small part of the solvent stock, either voluntarily, by means of a bleed stream, or involuntarily, by means of solvent losses caused by small leaks or improper operation. Thus the concentration of contaminants hardly ever rose too high. However, such practices are becoming less and less acceptable, both from an environmental and from an economic point of view. Operators are striving more and more to reduce the number of solvent changes. Whereas maybe twenty years ago solvent stocks would be replaced at the rate of two up to ten inventories per year, nowadays solvent stock replacements of less than once every year are aimed for. However, at such low replacement rates it becomes increasingly necessary to control the degree of degradation and contamination.
The applicant has carried out extensive investigations into the problem of contamination of alkanolamines and how to deal with it. It was found in the first place that a proper operation of the absorption and regeneration processes, avoiding excessive (localised) heat inputs and ingress of air, is fundamental for minimising the formation of degradation products. Particularly to be avoided are oxygen, elemental sulphur, hydrogen cyanide and carbon disulphide. Nevertheless degradation and contamination still may occur, so there remains a need for a cheap, simple and effective purification process.
It has been proposed to clean contaminated, degraded alkanolamines by various processes, but so far, none of these has become very popular. For instance electro-dialysis has been proposed and is even offered on a commercial scale. So far this technique has not had much success as the cost thereof is about equal to a complete replacement of the solvent stock, especially if the amount of solvent is relatively small. Furthermore, it is known generally that membranes are vulnerable, both physically and chemically, and prone to plugging.
Another technique which is offered on a commercial scale, is ion exchange. This technique has not met with much success either, as it appears that it removes only acid contaminants so leaves in the heavy, high molecular weight, degradation products. Moreover, the process is not cheap either and produces a lot of wastewater, which is undesirable and sometimes difficult to dispose off.
So currently in most plants, when purification of the alkanolamine is required and gradual bleeding and replacing is not desired, a simple distillation, also known as thermal reclaiming, is employed using available on-site equipment such as distillation columns. This is especially suited to batch reclaiming processes. Dedicated thermal reclaimers are also known, for continuous reclaiming of a slip-stream, when the production of contaminants is high due to the particular composition of the acid gas stream being treated. However, in order to recover the valuable components as much as possible, long residence times must be employed. That in turn results in cracking of the bottom product, and consequently, in contamination of the reclaimer top product with light cracked species. It is to be noted that even when partial refluxing or recirculation of the aqueous alkanolamine takes place in the conventional thermal reclaimers, the process still should be considered as a single step process.
In UK patent specification No. 1,572,682 it is proposed to use steam distillation to remove oxazolidone from alkanolamines, but the residence times, in combination with the temperature of about 200 to 300.degree. C. employed in that process, are unattractively high, resulting in the disadvantages explained above.
Neither do conventional vacuum distillation techniques using e.g. bubble-cap plate towers and tubular boilers fully satisfy, for the applicant found that also then unacceptable degradation may take place. Moreover, cost and heat economy remain problematic. Thus the general practice of trying to reclaim the aqueous alkanolamine in a conventional column, still or evaporator does not yield satisfactory results.